Research ArticleControllable Synthesis and Photocatalytic Activity ofNano-BiOBr Photocatalyst
Xiaoyang Wang, Fuchun Zhang , Yanning Yang, Yu Zhang, Lili Liu, and Wenli Lei
College of Physics & Electronic Information, Shaanxi Key Laboratory of Intelligent Processing of Big Energy Data, Yan’an University,Yan’an, 716000 Shanxi, China
Correspondence should be addressed to Fuchun Zhang; [email protected]
Received 19 November 2019; Accepted 22 January 2020; Published 24 February 2020
Nano-BiOBr photocatalysts were successfully prepared by hydrothermal synthesis using the ethylene glycol solution. The nano-BiOBr photocatalysts were characterized and investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM),photoluminescence (PL), and UV-vis diffuse reflectance spectroscopy (UV-Vis DRS), and the catalytic ability towardphotodegradation of rhodamine B (RhB) was also explored. The results showed that the crystallinity of the nano-BiOBrphotocatalyst decreased with the increase of the concentration, while it increased with the amount of the applied deionizedwater. The morphology of the nano-BiOBr photocatalyst changed from microspheres to cubes and then to a mixture ofmicrospheres and flakes with the increasing of the concentration and from microspheres to flakes with the addition of thedeionized water. The results indicated that the concentration and solvents have an essential influence on the bandgap energyvalues of the nano-BiOBr photocatalyst, and photocatalyst showed an excellent photocatalyst activity toward photodegradationof RhB. The degradation yields of photocatalyst decreased with the increase of the concentration and increased with theaddition of the deionized water. PL intensity of photocatalyst increased with the increase of the concentration and weakenedwith the addition of the deionized water.
1. Introduction
In recent years, the phenomenon of global water pollutionhas become a more and more severe issue with the rapiddevelopment of the economy, which has attracted wide-spread attention because of the close relationship betweenwater resources and people’s daily work and life [1, 2].Many ways can cause water pollution, one of them beingthe textile industry and wastewaters with organic dye, whichare challenging due to their poor biodegradability [3–5].Semiconductor bismuth halide (BiOX, X=Cl, Br, I)-basedphotocatalysts have attracted extensive attention fromresearchers because of their unique structure and excellentphotocatalytic properties [6, 7]. BiOBr was the target mate-rial of the presented investigations because of its moderatebandgap, open layered structure, high oxidation ability,indirect transition mode, high visible light response ability,and excellent stability [8, 9]. There are many methods to
prepare BiOBr, such as high temperature-based solid-state[10], hydrothermal [11], solvothermal [12], water- (alcohol-) based [13], ultrasound-assisted [14], and electrospinningmethod [15]. Among them, hydro- and solvothermalmethods are the most commonly used synthesis pathways.The structure, morphology, crystallinity, and phase forma-tion of the photocatalysts can be effectively obtained throughcontrollable synthesis because of the slow product formationrate, simple and easy ways to control reaction conditions, andstable reaction environment during the water- (solvent-)based thermal reaction [16]. For example, nano-BiOBrmicrospheres were synthesized previously by the solvother-mal method using ethylene glycol (EG) as a solvent [17]. Onthe other hand, nano-BiOX microspheres were obtainedusing other solvothermal methods and the same solventEG [18]. BiOBr/SrFe12O19 nanosheets were synthesized bythe solvothermal method using deionized water (DI) as asolvent [19]. AgBr/BiOBr nano-heterostructure-decorated
HindawiJournal of NanomaterialsVolume 2020, Article ID 1013075, 7 pageshttps://doi.org/10.1155/2020/1013075
polyacrylonitrile nanofibers were synthesized by electro-spinning technique and solvothermal treatment in the pres-ence of an EG solution as the reductant [20].
Therefore, the present paper is aimed at usingBi(NO3)·5H2O and CTAB as raw materials, with EG andDI as a solvent, under the condition of different concen-trations and different solvents to obtain nano-BiOBrphotocatalyst. The influences of different solvents and con-centrations of precursors on the structure, morphology,optical properties, and photocatalytic activities were alsoinvestigated systematically.
2. Experiment Section
2.1. Synthesis of Nano-BiOBr Photocatalyst. In the first step,2mmol of Bi(NO3)·5H2O is added to 80ml of EG, and thesolution was ultrasonicated until it was completely dissolved(obtaining solution A). Afterward, 2mmol of CTAB wasintroduced into solution A, stirred magnetically until it wascompletely dissolved (solution B). Next, solution B was intro-duced into a high-temperature reactor (the filling degree is80%), and after constant temperature reaction at 180°C for10 hours in an incubator, the solution was naturally cooledto room temperature, and the precipitate was separated.Finally, the precipitate was washed with DI and alcohol,and then, the nano-BiOBr photocatalyst was finally obtainedafter the drying procedure at 80°C for 12 hours. Table 1shows the abbreviation and synthesis parameters of nano-BiOBr photocatalyst obtained with different synthesis condi-tions, abbreviating them with (a)–(d) in the latter stages ofthe manuscript.
2.2. Characterization of Nano-BiOBr Photocatalyst. Thecrystalline phases were determined by a Bruker D8 AdvanceX-ray diffractometer (XRD) using a Cu Kα (λ = 0:15418 nm)radiation in the θ~2θ Bragg. The morphologies of the as-prepared samples were observed and investigated by a fieldemission scanning electron microscope (FESEM, NovaNanoSEM 450, FEI). The UV-vis absorption spectra of thesamples were measured with a Cary 5000 (Agilent, USA).The photoluminescence (PL) spectra were observed with aHe-Cd laser 280nm.
2.3. Determination of the Photocatalytic Activity of theNano-BiOBr Photocatalyst. The photocatalytic reactionswere carried out in a CEL-LAB500E4 multisite photochem-ical reaction system. The catalytic activity of the target deg-radation product was evaluated by using nano-BiOBrphotocatalyst under the visible light source. In a typicalphotocatalytic experiment, 0.05 g of the nano-BiOBr photo-catalyst was dispersed into 50ml of RhB (10mg/l) solutionand magnetically stirred in the dark for 30min to reach theadsorption-desorption equilibrium between RhB and thenano-BiOBr photocatalyst. Then, the light source was turnedon, and a sample of 4ml of the suspension was continuallytaken from the reaction cell at every 15 minutes and centri-fuged. Finally, the absorbance of the supernatant at the max-imum absorption wavelength was analysed through anultraviolet-visible spectrophotometer (UV-1901). The degra-
dation efficiencies were calculated according to the expres-sion of degradation rate (1 − A/A0), where A0 is theabsorbance of the target degradation at its maximum absorp-tion wavelength before illumination, and A is the absorbancevalue after illumination for a certain time.
3. Results and Discussion
3.1. XRD Analysis. The XRD patterns of nano-BiOBr photo-catalyst at different concentrations and different solvents areshown in Figure 1. It is clear that the prepared nano-BiOBrphotocatalysts were consistent with the standard diffractionpattern of tetragonal BiOBr (PDF#85-0862) (Figure 1(a)).No other specific diffraction peaks were detected, indicatingthat the nano-BiOBr photocatalysts prepared in differentconcentrations and different solvents are pure tetragonalnanoparticles. The intensity of the diffraction peak is weak-ened with the increase of the concentration, indicating thatthe concentrations are the key factor affecting the crystallin-ity of nano-BiOBr photocatalyst, 2-theta was right-shifted,and the crystal space d decreased according to the Pragueequation (d sin θ = nλ). The intensity of the diffraction peakincreases with the increase of DI, indicating that the increaseof DI throughout all the experiments is beneficial to increasethe crystallinity of nano-BiOBr photocatalyst; the influencefactors were more, and the main reason remains to be fur-ther studied.
In order to study the stability of the nano-BiOBr photo-catalyst, the XRD of the nano-BiOBr photocatalyst after thephotocatalytic reaction was investigated (Figure 1(b)). Com-pared with the results before the photodegradation, there areno noticeable changes in the crystal phases of the samples.
3.2. SEM Analysis. Figure 2 shows the SEM images of nano-BiOBr photocatalysts prepared using different concentra-tions and different solvents. As shown in Figure 2(a), BiOBrmicrospheres with a diameter of ~2μm were obtained, andthe BiOBr microspheres are self-assembled from irregularnanosheets with a thickness of ~10 nm. From Figure 2(b),BiOBr cubes of ~2μm are obtained, BiOBr being also self-assembled from irregular nanosheets in a fixed manner.Compared with (a) photocatalyst, (b) photocatalyst is self-assembled from nanosheets more densely, and the thicknessof the nanosheets is higher (~20nm). As shown inFigure 2(c), irregular sheetlike nano-BiOBr was obtained;compared with (b) photocatalyst, (c) photocatalyst is a nano-sheet with a thickness of ~15 nm. From Figure 2(d), it can beobserved that a mixture of BiOBr microspheres and sheetlikeBiOBr is obtained. BiOBr microspheres are tightly assembledfrom irregular nanosheets with a thickness of ~15 nm.
Table 1: The specific process parameters of samples.
Sample V1 : V2 CBi : CBr T (°C) Notes
(a) 8 : 0 1 : 1 180 V1: EG volume
(b) 8 : 0 1 : 3 180 V2: DI volume
(c) 8 : 1 1 : 3 180 CBi: the source of Bi+3
(d) 8 : 0 1 : 5 180 CBr: the source of Br-
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Compared with (b) photocatalysts, (d) photocatalysts havemore unassembled nanosheets, and the thickness of thenanosheets is smaller and assembled in a way that cubesbecome microspheres. The results showed that the mor-phology of the nano-BiOBr photocatalyst changed frommicrospheres to cubes and then to a mixture of microsphereand flakes with the increasing of the concentration of pre-cursors. Moreover, the morphology of nano-BiOBr photo-catalyst changed from microspheres to flakes with theaddition of DI.
3.3. UV-Vis DRS Analysis. Figure 3 shows the UV-vis diffusereflectance spectra and corresponding bandgap energies ofthe nano-BiOBr photocatalyst. It can be observed that theabsorption edges of (a)–(d) photocatalysts can be found at439, 450, 446, and 453nm, respectively, indicating that theabsorption wavelength range of (d) photocatalyst is the larg-est, and the absorption wavelength range of (a) photocatalystis the smallest, meaning that, more visible light can beabsorbed with the increasing of the concentration; theabsorbed visible light is reduced with the addition of DI.
(A)
(B)
(C)
(D)
Inte
nsity
(a.u
.)
0 30 60 902-theta (degree)
(a)
0 30 60 90
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.)
2-theta (degree)
(A)
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Figure 1: XRD patterns of samples (a) before and (b) after the reaction.
Figure 3: UV-vis diffuse reflectance spectra and a corresponding bandgap width of (A)–(D) photocatalysts.
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The position of the absorption edge is closely related to theforbidden bands of the semiconductor photocatalyst. Theforbidden bandwidth of the BiOBr photocatalyst is calculatedby Equation (1) [21].
αhv = A hv − Egð Þn/2, ð1Þ
where α, h, ν, A, and Eg represent the intrinsic absorptioncoefficient, the Planck constant, the frequency of light, theproportion constant of photocatalyst, and the bandgap widthof semiconductor, respectively. n = 2 for direct bandgapsemiconductor, and n = 4 for indirect bandgap semiconduc-tor. n = 4 because BiOBr photocatalyst is an indirect bandgapsemiconductor. Using the formula on the hν~ðαhνÞ1/2 curve,as shown in Figure 3(b), it can be observed that the tangent inthe middle section of the curve and the intercept betweentangent and abscissa is the bandgap of BiOBr photocatalyst.The bandgap of (a)–(d) photocatalyst measured by plottingand tangent fitting was 2.77 eV, 2.57 eV, 2.67 eV, and2.53 eV, respectively. It can be observed that the bandgapdecreases continuously with the increase of the concentrationand the bandgap increases with the addition of DI. It can beconcluded that the concentration of substances and solventshas an important influence on the bandgap energy values ofBiOBr photocatalyst.
3.4. Photodegradation of RhB Using Nano-BiOBr Semiconductors
3.4.1. UV-Vis Absorption Spectral Analysis. Figure 4 showsthe UV-vis absorption spectral changes of RhB aqueoussolution vs. irradiation time in the presence of the (a)–(d)photocatalysts. With the increase of the irradiation time,the absorption maximum of the spectra declined, andthe band shifted to a smaller wavelength, which indicatesN-demethylation and the destruction of the conjugatedstructure in the RhB photodegradation process [22]. Theblue-shifted band implies that the main photocatalyticdegradation path of the RhB is N-demethylation. The majorpeaks are reduced gradually during visible light irradiation,indicating a step-by-step degradation of RhB. The RhB UV-vis absorption spectra of the (a) photocatalyst were approxi-mately a straight line after 60min of illumination, indicatingthat the photocatalytic reaction of the (a) photocatalyst toRhB was mainly completed. The absorption peak of RhB inthe visible light region of (a), (c), and (d) photocatalysts hascompletely disappeared after 80min, and the decolourisationefficiency reaches 100%. The RhB solution photodegradedusing (b) photocatalysts had a weak absorption peak at theend of the process. The findings are in accordance with theresults obtained from previous sections, as the degradationefficiency of photocatalyst decreases with the increase of theconcentration. On the other hand, the degradation efficiencyof photocatalyst increased with the addition of DI.
3.4.2. Degradation Efficiency for RhB of BiOBr Photocatalysts.Figure 5 shows the degradation performance for RhB byphotocatalysts prepared in different concentrations and dif-ferent solvents. The results showed that the degradation per-
formance of photocatalyst declines in varying degrees withthe increase of the concentration, which indicates that theconcentration can change the degradation performance ofthe photocatalyst. The degradation performance of photoca-talyst improves with the addition of DI. The degradation per-formance of (a)–(d) photocatalysts to RhB was 96.3%, 68.8%,88.1%, and 81.3%, respectively, after 60min of light irradia-tion. It can be summarised that (a) photocatalyst showedthe highest photocatalytic activity for RhB.
3.5. PL Analysis. Nano-BiOBr photocatalyst using differentconcentrations and different solvents were characterized byphotoluminescence (PL) spectroscopy to verify further theconclusions mentioned above. The excitation wavelengthwas 280nm, and the emission range was 400nm to 600 nm,as shown in Figure 6. The movement, transfer, and recombi-nation rate of photogenerated electron-hole pairs wererevealed by PL spectra. The lower the intensity of emissionpeaks in PL spectra, the higher the separation efficiency ofelectrons and holes in semiconductors and the higher thephotocatalytic activity of photocatalysts that were observable[23]. It can also be seen that the PL intensity increases withthe increase of the concentration, while the photocatalyticactivity decreases. As can also be observed, the PL intensityis weakened with the addition of DI and the photocatalyticactivity increased. It can be seen from the figure that all ofthe photocatalysts have emission peaks at 437nm, 449nm,and 466nm and the order of luminous intensity is(b)> (d)> (c)> (a), which means that the recombination rateof electrons and holes of the four materials decreases gradu-ally and the photodegradation performance is graduallyenhanced, which is in accordance with the previous photo-catalytic experiment results.
4. Conclusion
Nano-BiOBr photocatalysts were successfully prepared bythe hydrothermal method. The influences of different con-centrations and different solvents on structure, morphology,optical properties, and photocatalytic properties are investi-gated systematically. The results show that the crystallinityof the nano-BiOBr photocatalyst decreases with the increaseof the concentration and increases with the increase of theDI. The morphology of the nano-BiOBr photocatalystchanges from microspheres to cubes and then to a mixtureof microsphere and flakes with the increasing of concentra-tion, from microspheres to flakes with the addition of DI. Itwas also proved that the concentration and solvents havean essential influence on the bandgap energy values of thenano-BiOBr photocatalyst. The degradation performance ofphotocatalyst with the decline in the increase of concentra-tions improves with the addition of DI. The semiconductor(a) showed the highest photocatalytic activity toward RhB.PL intensity of photocatalyst increased with the increase ofthe concentration and weakened by the addition of DI.Therefore, the high photocatalytic activity of photocatalystsfor contaminant aqueous solution makes this research anew platform to develop flexible photocatalyst for practicalapplications in water purification.
5Journal of Nanomaterials
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Data Availability
The data used to support the findings of this study areavailable from the corresponding author upon request.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
The project was supported by the National Natural ScienceFund of China (61664008), the Scientific and TechnologicalInnovation Team (2017CXTD-01), the Scientific ResearchProgram Funded by Shaanxi Provincial Education Depart-ment (18JK0870), the Yan’an Science and TechnologyResearch and Development Program (2018KG-01), theYan’an University Ph.D. Research Project (YDBK2018-39),and the Yan’an University Industry-University-ResearchCooperation Project (YDCXY2018-07).
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